Just thought I would post what I just figured out. I used the leaked pictures of the 645d and the tripod treaded mount which is a known size and measured the sensor to be 36mm x 27mm. It makes sense that the normal lens of the 645d is going to be 55mm. Someone else had figured the difference to be a crop of 1.36 just by lens size and it equals out in the measurements. So here you have it - a $10k full frame camera... Just $2k more than the FX frame D3x Nikon. Mazel Tov!!!

Just thought I would post what I just figured out. I used the leaked pictures of the 645d and the tripod treaded mount which is a known size and measured the sensor to be 36mm x 27mm. It makes sense that the normal lens of the 645d is going to be 55mm. Someone else had figured the difference to be a crop of 1.36 just by lens size and it equals out in the measurements. So here you have it - a $10k full frame camera... Just $2k more than the FX frame D3x Nikon. Mazel Tov!!!

43mm on 24X36 are "normal", but also 50mm and 55mm...
Pentax say Medium Format and 55mm lens, so i guess it´s like
43mm on 24X36(that´s the diagonal of 24X36mm).
So 43 X 1,33 = 57mm(close to 55mm).
645 has 75mm and 24X36 has 50mm normal lens,
so 75 - 50 = 25 and half of 25 are 12,5 then 50mm + 12,5 = 62,5mm.
That is between 645 and 24X36, but if it is 43mm lens for 24X36..
75 - 43 = 32 and half of 32 are 16 then 43 + 16 = 59mm.

Or diagonal for 645 are 70mm and for 24X36 43mm.
Then we have 70 - 43 = 27 and half of 27 are 13,5,
and 43 + 13,5 are 56,5mm.There it is, almost a 55mm normal lens.
And the diagonal are in the middle of 24X36 and 645(~43X56mm).

I used the leaked pictures of the 645d and the tripod treaded mount which is a known size and measured the sensor to be 36mm x 27mm. It makes sense that the normal lens of the 645d is going to be 55mm. Someone else had figured the difference to be a crop of 1.36 just by lens size and it equals out in the measurements.

Leaked pictures? I guess you are not referring to the pictures announced by Pentax for PIE or to pictures of the prototype at this event? If so, where did you actually see the sensor to be able to measure it?

Maybe u used older pictures from 2007, whith open back and a 18MP-sensor prototype , right?

My point was obviously missed... To put a cropped sensor on one type of camera is fine and another is unspeakable. I gave what I believed to be the size of the sensor earlier and because I didn't give a "square root of my apartment" explanation I was discounted. Maybe next time I'll try to use super string theory to explain the benefits of 14bit raw... Really, I don't give 2 shits about the size of the sensor, I just presented what I found.

My point was obviously missed... To put a cropped sensor on one type of camera is fine and another is unspeakable. I gave what I believed to be the size of the sensor earlier and because I didn't give a "square root of my apartment" explanation I was discounted. Maybe next time I'll try to use super string theory to explain the benefits of 14bit raw... Really, I don't give 2 shits about the size of the sensor, I just presented what I found.

you just stirred up the FF moaning club... Anyways to be brief, there would be little sense in putting a FF sensor in anything but an APS-C body, since, for all intents and purposes it IS a FF body. Now you could argue a bit re: the possibility of needing more "room" and a larger mount for anti shake BUT it's not likely. Besides unless you make an adapter for a smaller mount to fit in a larger and give it the registration distance it needs you eliminate all the old FF lenses by designing such a HUGE registration distance. In effect you have a ff camera that can only use medium format lenses...
No FF sensor (though I thought they could do it as an option (IF they had interchangeable sensors) that is unlikely at this point. Cropped MF sensor is th most likely opinion and all we need to see if MP count and the real dimensions..

Well, I did consider that and though I don't actually own a 645, I do own a view camera and it has a 1/4" mount. I took it for granted that if a view camera with a span of almost 30" and the shear weight (it's metal, not composite) only required a 1/4" mount, why would a modern digital that weighed less than a tenth of the weight require a 3/8" mount? It could, but really the only time I have seen the need for a 3/8" stud was on a light stand. But, then again here we are with common sense - which doesn't usually fly around here... Now, if I had a protractor, a slide rule and the solution to Fermat's Last Theorem, maybe... maybe... or maybe monkeys will start flying out of my ass....

You know what - screw it, I'm changing my assessment. I'll admit I measured wrong - I really came up with 16cm x 12cm - about the size of my sack... Super sized MF!!! I also used Fred Miranda's SI Pro to blow up that picture and count each and every photo site on that damn chip - it was 88mp!!! Holy Crap!!! Aren't you guys happy now??? Imagine the pictures you could take with that!

Pointless discussion.
It is known (see the Luminous Landscape) that the latest 645D prototypes uses the Kodak 31.6MP medium format sensor, while the first ones started with the now discontinued 18MP sensor. Those are not full-sized 645 sensors, but they're not small format, either. Any measurements which "proves" the contrary are erroneous.
And even the common sense tells us, why put a small format sensor into an expensive 645 body when you can use a K-mount body instead? You get no advantage from doing this; instead you'll make the camera unnecessarily big, drastically restrict the lens choices while you'll have to say bye-bye to ultrawides. At 10000$, it would be very much overpriced, as well - a commercial failure, without a doubt.
Honestly, you just want Pentax to shoot themselves in the foot. Give up, they're smart enough not to do it.

Well, I did consider that and though I don't actually own a 645, I do own a view camera and it has a 1/4" mount. I took it for granted that if a view camera with a span of almost 30" and the shear weight (it's metal, not composite) only required a 1/4" mount, why would a modern digital that weighed less than a tenth of the weight require a 3/8" mount? It could, but really the only time I have seen the need for a 3/8" stud was on a light stand. But, then again here we are with common sense - which doesn't usually fly around here... Now, if I had a protractor, a slide rule and the solution to Fermat's Last Theorem, maybe... maybe... or maybe monkeys will start flying out of my ass....

For the record, my Mamiya 645E (bought new back in 2000 or so) had a 3/8" mount, but came with an insert to take it down to 1/4". I've seen the 3/8" one referred to as "Euro thread" by Manfrotto (at least that is what it says on one of my mount plates).